Piezoelectric apparatus



-1 1937. s. A. BOKOVOY ET AL 3 PIEZOELECTRIC APPARATUS Filed Nov, 30, 1935 it M13 Wm- M12 Ina 621207 5 V/l/l/ll/I/Af 5 5 C13 E12 mod FZCLZD W5 Patented Dec. 14, 1937 UNITED STATES PATENT OFFICE PIEZOELECTRIC APPARATUS ware Application November 30, 1935, Serial No. 52,304

8 Claims.

This invention relates to apparatus for use in connection with piezo-electric elements and has special reference to the mounting of piezo-electric quartz crystal oscillators and resonators.

Numerous mounting arrangements for piezoelectric elements are known in the art. Regardless of the advantages claimed for each type it may be said generally that none permits that optimum performance which is only achieved when the piezo-electric crystal is operated close to its elastic limit. Attempts to operate a piezoelectric crystal close to its elastic limit have heretofore resulted in failure due to voltage breakdown between one or all of the electrode plates and the crystal. The corona discharge or areing, characteristic of such breakdown may either puncture the crystal or, in time, badly disfigure both the crystal and the electrodes associated therewith. In some cases a noticeable deposit in the form of either a wet or a dry oxide may appear upon the crystal, and its electrodes, causing the vibratile element to oscillate less strongly, or to cease operation entirely. Thorough cleansing of the electrode surfaces and the crystal may permit of normal oscillation though if it is necessary to regrind the crystal its original frequency characteristics may be lost. This undesired phenomenon which is a usual concomitant to the operation of piezo-electric elements mounted in known ways is especially pronounced at higher frequencies, so much so that where ultra-high frequency quartz crystals are employed arcing occurs long before the elastic limit of the quartz is reached. The described phenomena, however, may occur with crystals designed to oscillate at frequencies as low as say 2000-4000 k. 0. Some of the factors determining breakdown are (1) the manner in which the crystal is cut, i. e. whether it is X-cut, Y-cut, V-cut, AT-cut, etc. (2) the frequency at which it is designed to oscillate (3) the type of holder, i. e. pressure type, air-gap, gravity type, etc. (4) electrode-crystal spacing (5) departure from paralleism of the crystal faces (6) flatness of the crystal spaces (7) smoothness of the crystal faces (8) flatness of the electrode surfaces (9) smoothness of the electrode surfaces (10) the grain of the material of which the electrodes are formed (11) the plate voltage (12) the radio frequency voltage applied to the crystal and (13) the type of circuit employed.

A principal object of the invention is to in crease the effective amplitude of vibration achievable with piez'o-electric crystals, whereby more power may be derived from a crystal of a given elastic limit.

Another object of the invention is to obviate arcing and corona-discharge between a piezoelectric element and its associated electrodes.

Another object is to increase the voltages which may be applied to piezo-electric quartz elements.

Another object is to prevent deterioration of the crystal and the formation of oxides on its electrode surfaces.

- Other objects of the invention are to ensure rapid starting, and long and safe operation for piezo-electric crystals.

The above objects and others ancillary thereto are accomplished in accordance with the invention by increasing the dielectric constant normally present between crystal and electrode surfaces and, where necessary or desirable, by increasing the flash-over distance between said surfaces.

Considered from one aspect the present invention is predicated upon a proper appreciation of the fact, and its application to the piezo-electric art, that when the air space between two armatures of an air-condenser is filled with a substance (such as mica) having a high dielectric constant, the high frequency voltage applied thereto can be greatly increased before breakdown occurs.

Preferably the dielectric material selected in carrying the invention into effect has a constant higher than that of the particular piezo-electric material to which it is applied, in any event it must have a dielectric constant higher than the air which it replaces. Among the dielectric substances which may be employed are varnishes, vitreous enamels, lacquers, metallic oxides, fused coatings of natural orsynthetic resins, thin sheets of glass or of fused quartz, cellulose esters and ethers, Cellophane, rubber coatings applied by electro-deposition, rubber sheeting, rubber containing resin and other fillers, mica. The material selected may be placed between the vibratile element and one electrode, or it may be placed adjacent or contiguous the major faces of the element. If desired they may be made an integral part of the electrode plates, for instance by oxi dizing, varnishing or otherwise coating the surfaces of the electrode plates.

Usually the application of such materials between the crystal and its electrodes or to the electrodes will alter the fundamental frequency of the piezo-electric element-the exact departure from normal depending upon the type of mounting and to some extent upon the type and quantity of the dielectric material employed. If the departure is such as to be intolerable the crystal may be ground or lapped to a frequency less than the required frequency to compensate for such departure.

Fig. 1 is a view in perspective of a structure embodying the invention showing a rectangular crystal plate adapted to be clamped adjacent its four corners. Fig. 2 is a sectional View taken on the line 2-2 of Fig. 1.

Fig. 3 is a perspective view and Fig. 4 a secmounting-including a pair of flat surfaced electrodes, E9-E'9 and crystal C9 the layers of intional View of the crystal and electrode plates similar to those 'of Fig. 1, showing another embodiment of the invention.

Fig. 5 is a top plan view of one type of center mounting. Fig. 6 is a sectional view taken on the line i'fi of Fig. 5.

Fig. 7 is a cross sectional View showing two plane-surfaced rectangular electrodes, a piezoelectric element and two dielectric layers assembled in accordance with the invention.

Fig. 8 is a perspective view of an electrode designed for nodal-zone 'mounting and Fig. 9 is a similar view of a pre-fabricated member of dielectric material adapted to be superimposed up'cnthe electrode of Fig- 8.

Fig.10 is a perspective view of a plane-surfaced electrode having 'a dielectric material coated thereon in accordance with the invention.

Fig. 11 is a cross sectional view of a simple gravity air-gap mounting embodying the invention.

Figs. 1 and 2 show one manner of applying the invention to a conventional air-gap pressure mounting of the type wherein the electrodes Ei andEZ are provided, respectively, with risers Tl, r3, r5, riprZ, r4, r6, etc., each adapted to contact the crystal element C adjacent one of its four corners. In this embodiment of the invention flat sheets of mica or other insulating ma.-

terial Mi-M2, having their edge portions cut away to accommodate the risers rl', r2, etc., are simply fitted into the air-gaps, in the central portions of the electrdes-the risers contacting the crystal adjacent its four corners as before. As in the usual arrangement clamping pressure, symbolically indicated by the vertical arrows, may be. applied to the assembly.

Figs. 3 and 4 employ the same similarly desig nated electrodes E! and E2; here, however, the insulating material, in addition to filling the airgap in the central portion of the electrode surfaces, is interposed between the risers Tl, r2, etc. The insulating material is conveniently in the form of separate sheets Ml M3, M2 M4. The rectangular sheets M2 M4 which span the entire area of the crystal are less rigid than the metal constituting the electrodes and serve to ,cushion the clamping pressure to a useful degree.

Figs. and 6 show the invention as applied to a center mounting wherein the upper electrode E5 and the lower electrode E5 circumscribe an area less than that of the total major surfaces of the crystal C5. The insulating strips M5 and MS span an area on the crystal faces greater than that embraced by the electrode surfaces, whereby the arc-over distance, designated 03?, between the electrodes and crystal is longer, (and hence the applied voltages may be'higher). than that obtaining when the insulating material is confined to the midsection of the crystal.

Referring to Fig. '7, it may be here noted that a characteristic feature of air-gap, and air-gap V 5 pressure holders, is: the. smaller the airgap, the

higher the radio-frequency voltage which may be applied to the electrodes before breakdown occurs. The ultimate condition is reached where the electrodes are in contact with the crystalf since theoretically at least, there is no ionizable substance between the charged surfaces to facilitate the formation of an arc. Arcing may, however, occur adjacent the edges of the electrodes and crystal, especially if, as is usually the case, the assembly is mounted within a confined space. Accordingly, in applying the invention 7 to a sulating material M9M9 are principally designed to eliminate external arcing. 'This is simply accomplished in accordance with the invention by providing layers of insulating material an electric arc. When so mounted the inner body ofthe insulating material serves principally to lessen the damping effect normally present due to direct contact of the inelastic electrode plates. I

Fig. 8 shows an electrode of the type disclosed in copending application Serial Number 13,887 to Samuel A. Bokovoy, filed March 30, 1935 (R; C. V. Docket 4038). This electrode, Eli], is pro-. vided with-a pair of oppositely located wedge.- shape risers ill-l8 adapted tocontact the crystal (not shown) at spaced points along'its nodal axis. Fig. 9 shows a pre-fabricated sheet ofinsulating material Mil having correspond? ing cut away portions Ii.l i enabling it to be superimposed upon this plate Ell]. When assembled together with the crystal the mounting ensures maximum rigidity with minimum damping effect due to clamping.

-large enough to extend outwardly a distance sufficient to arrest the formation or passage of Fig. 10 shows a rectangular electrode plate El2 with the insulating 'material MIZ intimately bonded thereto. The material may be a fusedon vitreous enamel, for example, or it may be of any other suitable material, having therequired dielectric properties, applied in any convenient manner. As indicated in the drawing the insulating coating M|2 may, and preferably does, coat the side edges of the electrode.

Fig. 11 shows the invention as applied to a gravity or simple air-gap holder wherein the crystal CIB is simply laid without clamping pressure on the insulating surface Ml2 of one of a pair of electrodes EI2, EH3, which may both 0011? veniently be similar in all respects with that described in connection with Fig. 12. Means, symbolically indicated by the arrows, are preferably provided to vary the electrode-crystal spacing. In addition to permitting the crystal to be operated close to its elastic limit such construction ensures substantial constant frequency of oscillation.

As a number of possible embodiments may be made of the above invention, and as changes may be made in the embodiments described, without departing from the spirit or scope of the invention it is to be understood that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the spirit of the appended claims and by the prior art. V

What is claimed is:- V I l. The combination-with a piezo-electricele ment having a relatively high fundamental period of vibration, of a pair of electrodes therefor and a layer of insulating material having a dielectric constant greater than air removably interposed between said element and each of said electrodes.

2. The invention as set forth in claim 1 wherein said insulating material is integral With at least one of said electrodes.

3. The invention as set forth in claim 1 where-- in said layer of insulating material extends beyond the edges of said piezo-electric element.

4. The combination with a piezo-electric element of a pair of metal electrode plates, each having a raised electrode-surface-portion, a layer of insulating material removably interposed between said element and the non-raised area of each of said electrode surfaces, and means for applying clamping force to said piezo-electric element through said raised portions of said electrode surfaces.

5. The invention as set forth in claim 4 wherein the raised electrode surface portions of said plates contact said piezo-electric element.

6. The invention as set forth in claim 4 wherein said insulating material is likewise removably interposed between the piezo-electric element and the raised portions of said electrodes, Whereby said clamping force is applied to said element through the insulating material contiguous said raised portions of said electrodes.

7. An electrode for piezo-electric elements comprising a metal plate having a layer of insulating material intimately joined to an electrode surface thereof.

8. The invention as set forth in claim 7 Wherein said insulating material covers at least that portion of the sides of said plate contiguous said electrode surface.

SAMUEL A. BOKOVOY. PAUL D. GERBER. 

